smith removes Xattrs such as security.capabilities from files

[astromechza]

When packaging an image, Smith does not copy across extended attributes on the files.

This means that users that need capability bits for running an image as non-root need to apply these retroactively to the image in new layers.

To fix, I assume we need some work in pack.go to copy across the PAXRecords from the file info to the tar headers in a similar way to https://golang.org/src/archive/tar/common.go?s=22088:22153#L619